Assyrian religion was an evolution of the ancient polytheistic Sumerian and Akkadian religions into henotheism, a religion based on the worship of one supreme god, but recognizing the existence of others. This was represented through the gradual takeover by Ashur of the roles of other gods, and this process runs parallel with the expansionist policies of the Assyrian Empire.[11] As the Assyrians extended their domain over other lands, they considered it important that the local peoples acknowledge the Assyrian king as the king of their lands as well. However, kingship at the time was linked very closely with the idea of divine mandate.[12] The Assyrian king, whilst not being a god himsef, was acknowledged as the chief servant of the chief god, Ashur. In this manner, the king's authority was seen as absolute so long as the high priest reassured the peoples that the gods, or in the case of the henotheistic Assyrians, the God, was pleased with the current ruler.[12] For the Assyrians who lived in Assur and the surrounding lands, this system was the norm. For the conquered peoples, however, it was novel, particularly to the people of smaller city-states. In time, Assur was promoted from being the local deity of Assur to the overlord of the vast Assyrian domain,[12] with worship being conducted in his name throughout the lands of the Assyrians. With the worship of Assur across much of the Fertile Crescent, the Assyrian king could command the loyalty of his fellow servants of Assur.

Ashur, the patron deity of the city of Assur from the Late Bronze Age, was in constant rivalry with the patron deity of Babylon, Marduk. In Assyria, Ashur eventually superseded Marduk even in his role as the husband of Ishtar.

^Parpola, Simo (1999). "Assyrians after Assyria" (in English) (HTML). Assyriologist. Journal of Assyrian Academic Studies, Vol. XIII No. 2,. http://www.nineveh.com/Assyrians%20after%20Assyria.html. "The gods Ashur, Sherua, Istar, Nanaya, Bel, Nabu and Nergal continued to be worshiped in Assur at least until the early third century AD; the local cultic calendar was that of the imperial period; the temple of Ashur was restored in the second century AD; and the stelae of the local rulers resemble those of Assyrian kings in the imperial period."